Seismic waves travel fast, on the order of kilometers per second (km/s). Here's an example to illustrate the difference: if two earthquakes occurred at the same place but exactly 24 hours apart, the wave travel times would be the same but the arrival times would differ by one day. The arrival time is the time when we record the arrival of a wave - it is an absolute time, usually referenced to Universal Coordinated Time (a 24-hour time system used in many sciences). that the wave took to complete its journey. Travel time is a relative time, it is the number of minutes, seconds, etc.
travel time = (distance from earthquake to seismometer) / (seismic wave speed) Faster waves will travel the distance quicker and show up on the seismogram first. To apply those ideas to earthquake studies, think of the earthquake location as the starting point for the trip and the seismometer as the place where the trip concludes. The mathematical formula we use in this problem is driving time = (distance of trip) / (driving speed)
If you have to travel 120 miles and you drive 60 mph, you'll get to your destination in two hours, if you are forced to drive at a speed of 30 mph, it will take you twice as long to arrive at your destination. Travel times are best conceptualized of with an analogy of an auto trip. The latter two are called surface waves they the travel along Earth's surface and their amplitude decreases with depth into Earth.
The first two wave types, P and S, are called body waves because they travel or propagate through the body of Earth. We'll go through each wave type individually to expound upon the differences. Seismic waves can be distinguished by a number of properties including the speed the waves travel, the direction that the waves move particles as they pass by, where and where they don't propagate. Also with increasing distance from the earthquake, the waves are separated apart in time and dispersed because P, S, and surface waves travel at different speeds. At farther distances the amplitude of the seismic waves decreases as the energy released by the earthquake spreads throughout a larger volume of Earth. Even in large earthquakes the intense shaking generally lasts only a few tens of seconds, but it can last for minutes in the greatest earthquakes. These are the waves that do the most damage to our buildings, highways, etc. Near an earthquake the shaking is large and dominated by shear-waves and short-period surface waves. The are many different seismic waves, but all of basically of four types:Īn earthquake radiates P and S waves in all directions and the interaction of the P and S waves with Earth's surface and shallow structure produces surface waves. An earthquake is a more complicated process than a stone splashing into water, and the seismic waves that are set up during an earthquake are more varied than those on the pond. You can picture this concept by recalling the circular waves that spread over the surface of a pond when a stone is thrown into the water. Seismic waves are propagating vibrations that carry energy from the source of the shaking outward in all directions. When you look at a seismogram the wiggles you see are an indication that the ground is being, or was, vibrated by seismic waves.